RELATIONSHIP OF HORIZONTAL EDDY DISPLACEMENTS TO MEAN MERIDIONAL MOTIONS IN THE STRATOSPHERE

Equivalent-barotropic integrations that have been carried out under hi gh resolution reveal organized subsidence inside the polar-night vorte x. The potential temperature theta along a material surface, which is a prognostic variable in the equivalent-barotropic system, drops to sh arply lower values across the edge of the vortex. Resembling observed motion in the Brewer-Dobson circulation, this behavior results from di abatic effects when the vortex is driven out of radiative equilibrium by wave advection. Lagrangian analyses carried out for ensembles insid e and outside the vortex elucidate specific thermodynamic processes wh ich act on individual bodies of air and are ultimately responsible for the Brewer-Dobson circulation. When the vortex is displaced out of zo nal symmetry, individual air parcels are driven out of thermodynamic e quilibrium with their surroundings. As a result, they experience irrev ersible heat transfer that leads to a hysteresis and net change of the ta with each complete orbit about the pole. Successive orbits then pro duce a drift of air to lower theta. Vertical motion estimated from ens emble-mean properties corresponds to an average descent rate of about 1 mm/s, which is in qualitative accord with estimates derived elsewher e. Within the Lagrangian framework, the disturbed circulation function s as a radiative refrigerator by converting work performed at its lowe r boundary into heat that is eventually rejected to space through long wave cooling. The Lagrangian analyses suggest a similar analog for pho tochemical considerations. Driven out of photochemical equilibrium, th e disturbed circulation can then function as a chemical engine by prod ucing ozone and transferring it to the extratropical lower stratospher e.